Laser beam source

ABSTRACT

In a laser beam source for projecting laser pulses, having a laser-active medium ( 10 ), a controllable pump device ( 11 ) for exciting the medium ( 10 ), a resonator ( 13 ) for inducing oscillation of the medium ( 10 ), which resonator has an end mirror ( 14 ) and an output mirror ( 15 ) with an orientation axially parallel to the medium ( 10 ), for the sake of projecting laser pulses of constant pulse energy immediately upon release of the laser beam source, one of the resonator mirrors ( 14 ) is pivotably supported and is pivoted out of its resonator position and back into its resonator position by an actuator ( 19 ) at selectable times (FIG.  1 ).

PRIOR ART

[0001] The invention is based on a laser beam source for projectinglaser pulses, in particular for machining material, as genericallydefined by the preamble to claim 1.

[0002] Such laser beam sources, embodied for instance as flash lamp- ordiode-pumped solid-state lasers, are used among other purposes inmachining material, for instance for welding, drilling or cutting (H.Hügel, “Strahlwerkzeug Laser”, Teubner-Verlag, Stuttgart, 1992). In it,a controllable shutter following the laser beam source and used forprotection purposes is synchronized with the control of the pump devicein such a way that first the shutter opens, and then thepulse-controlled flash lamps or diodes excite (pump) the laser-activemedium, which in the case of the solid-state laser is the laser crystal.This sequence is maintained in order to prevent the closed shutter frombeing burned by the laser pulses, but as a consequence of it, in thefirst laser pulses projected after the activation of the pump devicethere are fluctuations in the pulse energy, which are caused by thethermal transient-response process of the laser-active medium. Thesefluctuations in energy have a very adverse effect on the machiningprocess, especially in single-pulse machining, for instance in weldingor drilling.

ADVANTAGES OF THE INVENTION

[0003] The laser beam source of the invention for projecting laserpulses, having the characteristics of claim 1, has the advantage thatthe laser-active medium, such as the laser-active active crystal, ispermanently excited and is thus thermally in equilibrium, by pivotingoutward of the resonator mirror from its resonator position, but thelaser oscillation is interrupted and thus no laser pulse is generated.As soon as laser pulses are to be projected, then by pivoting theresonator mirror back into its resonator position, in which the normalto the mirror surface is aligned with the normal to the surface of theother resonator mirror or is oriented parallel to it, the lasing processof the medium is enabled. All the laser pulses projected thus—since evenwhen the first laser pulses are projected, the thermaltransient-response process has already been concluded—have the samepulse energy, and it is assured that as a result, the outcome ofmachining, from the very outset, meets the quality desired.

[0004] The laser beam source having the characteristics of claim 2 hasthe same advantages. Once again, with the shutter disposed between thelaser-active medium and one of the resonator mirrors, the process oflaser oscillation between the resonator mirrors is blocked by theclosure of the shutter and set into motion by the opening of theshutter.

[0005] By the provisions recited in the other claims 3-6, advantageousrefinements of and improvements to the laser beam sources defined byclaim 1 and claim 2, respectively, are possible.

[0006] For interrupting the lasing process, only small pivot angles ofthe pivotably supported resonator mirror are needed, so that in anadvantageous embodiment of the invention, a piezoelectric actuator canbe used as the actuator. The actuator and the pivotable resonator mirrorare advantageously coupled in such a way that when the piezoelectricactuator is not excited, the lasing process is interrupted, or in otherwords the resonator mirror is pivoted out of its resonator position, andthat whenever laser pulses are required, the piezoelectric actuatorexcited via the control unit pivots the resonator mirror back into itsresonator position.

[0007] The laser beam source of the invention for projecting laserpulses having the characteristics of claim 7 has the advantage that thelaser beam source is always in full operation, and only the beamdirection of the laser pulses is shifted, so that in pauses duringmachining the laser pulses do not reach the machining site.

[0008] By the provisions recited in the further claims 8-11,advantageous refinements of and improvements to the laser beam sourcedefined by claim 7 are possible.

[0009] In one advantageous embodiment of the invention, the deflectordevice is embodied as a deflector mirror that can be transferred by anactuator into two pivoted positions; the orientation of the deflectormirror in its pivoted positions is accomplished such that the beamdirection of the laser pulses in one pivoted position is aimed at amachining site and in the other pivoted position is aimed at a so-calledbeam sump. The energy of the laser pulses is dissipated in the beamsump. Once again, the requisite pivot angles of the deflector mirror arerelatively small, so that a piezoelectric actuator can be used as theactuator.

[0010] In an alternative embodiment of the invention, the deflectordevice is embodied as an acousto-optical modulator, which by applying ahigh-frequency acoustic power to a so-called Bragg cell diffracts thelaser pulse passing through the Bragg cell to the first order and thuspivots it away from the machining site.

DRAWING

[0011] The invention is explained in further detail in the ensuingdescription in terms of exemplary embodiments shown in the drawing.Shown are:

[0012] FIGS. 1-4, a block circuit diagram of a laser beam source, infour different exemplary embodiments, respectively.

DESCRIPTION OF THE EXEMPLARY EMBODIMENTS

[0013] The laser beam source shown in FIG. 1 for projecting laserpulses, which is used particularly for machining material, has alaser-active medium 10 in a known manner, which by being supplied withenergy is transferred from a state of thermodynamic equilibrium into alaser-active state. The energy delivery is effected by means of acontrollable pump device 11, which is clocked by a control unit 12; thenumber of pulses, pulse frequency and pulse length of the laser beamsource can be varied by means of the clocking rate and the length of thecontrol pulses. In so-called solid-state lasers, for instance the mostimportant representative of the category in industrial materialmachining, which is the Nd:YAG laser, the laser-active medium 10 isachieved with a crystal, which is doped with metal ions or rare earthions as a laser-active medium. In the Nd:YAG laser, the so-called hostcrystal is an yttrium-aluminum garnet, in which Y ions in the crystallattice are replaced with Nd (neodymium) ions. The pump device 11 can bea flash lamp or a diode, which is controlled in pulsed form andilluminates the solid-state body or crystal. By means of a resonator 13,which in the simplest case comprises a totally reflective end mirror 14and a partially permeable output mirror 15, some of the excitationenergy made available by the pump device 11 is out-coupled by the lasereffect as electromagnetic radiation in the form of laser pulses. Thetask of the resonator 13 is to send the laser light through thelaser-active medium 10 multiple times, as a result of which anamplification process ensues, and the laser lases; that is, laseroscillation begins. In this state, an equilibrium is established betweenthe energy delivered to the laser-active medium 10 and the energy outputin the form of laser radiation by the output mirror 15. A so-calledshutter 16 is also located behind the output mirror 15 in terms of thebeam direction; it has a protective function and is closed when thelaser beam source is turned off. Upon actuation of the laser beamsource, the shutter 16 is opened by the control unit 12. A diaphragm 21with an aperture 22 disposed between the laser-active medium 10 and theoutput mirror 15 serves to improve the beam quality of the laserradiation.

[0014] To prevent the shutter 16 from being burned by laser pulses, thecontrol of the shutter 16 is harmonized with the control of the pumpdevice 11 in such a way that first the shutter 16 opens, and then thepump device 11 pumps the laser-active medium 10. To prevent theprojection of laser pulses with fluctuations in pulse energy during thethermal transient-response process, which ensues at the onset ofpumping, in the laser-active medium 10, which is not yet in thermalequilibrium, blocking means for suppressing the lasing process of thelaser-active medium 10 are disposed in the resonator 13, and they can bedeactivated at selectable times for a selectable length of time. Thetimes for deactivation of the blocking means are set such that thethermal transient-response process of the laser-active medium 10 isconcluded, and this medium has reached its thermal equilibrium. Theselected length of time depends on the duration of machining. Thedeactivation advantageously occurs between two lighting pulses by theflash lamp or diode.

[0015] In the exemplary embodiment of FIG. 1, the blocking means have apivot bearing 17 for the end mirror 14 in the resonator 13 and anactuator 18, which adjusts the pivotably supported end mirror 14 andwhich is embodied here as a piezoelectric actuator 19 controlled by thecontrol unit 12. As long as the lasing process of the laser-activemedium 10 is to remain suppressed despite pumping by the pump device 11,or in other words as long as the blocking means are operative, the endmirror 14 assumes its position shown in dashed lines in FIG. 1, in whichit is pivoted out of its resonator position. The resonator position isdefined in that the normals of the two resonator mirrors 14, 15 arealigned with one another or oriented parallel to one another, so that awave train of the electromagnetic waves emitted spontaneously in thelaser-active medium 10 passes through the medium 10 multiple times byreflection from the two resonator mirrors 14, 15 and is amplifiedbecause of induced emission until such time as so-called laseroscillation ensues. If the end mirror 14 is transferred to its positionshown in dashed lines in FIG. 1, then the wave train reaching it,emitted in the axial direction by the laser-active medium 10, isreflected in a different direction and does not arrive back at themedium 10. The amplification process cannot ensue in that case, and thelaser cannot lase.

[0016] If a laser pulse train is required for machining material, thenthe control unit 12 triggers the piezoelectric actuator 19, and thepiezoelectric actuator pivots the end mirror 14 into the resonatorposition shown in solid lines in FIG. 1, in which the amplificationprocess described above ensues, and the laser beam source projects thepulse train. The control of the piezoelectric actuator 19 is effectedbetween two successive lighting pulses of the flash lamp or diode, sothat the return process of the end mirror 14 is concluded once thesecond lighting pulse has been projected. Since only the lasing processof the medium has been blocked, but not the pumping of the medium 10 bythe pump device 11, the medium 10 is in thermal equilibrium when the endmirror 14 pivots into its resonator position, and all the laser pulsesprojected by the laser beam source have the same, constant pulse energy.

[0017] It is understood that is possible, instead of the end mirror 14,also to provide the output mirror 15 with a pivot and to couple it withthe piezoelectric actuator 19. The mode of operation is the same as thatdescribed above.

[0018] In the exemplary embodiment of FIG. 2, the blocking means forsuppressing the lasing process of the laser-active medium 10 have ashutter 20 and an actuator 18 that is actuated by the shutter 20. Theactuator 18 is in turn embodied as a piezoelectric actuator 19, which istriggered by the control unit 12 for opening and closing of the shutter20. The shutter 20 disposed between the output mirror 15 and thelaser-active medium 10 likewise, in the closed state, prevents thelasing of the medium 10, since because of the shielding of the outputmirror 15 from the medium 10, the above-described amplification andoscillation process cannot ensue. Not until the shutter 20 is opened arethe lasing conditions for the laser established, and the laser beamsource projects the laser pulses, whereupon even the first laser pulsehas the same pulse energy as those that follow it.

[0019] The shutter 20 can be embodied in various ways. In the exemplaryembodiment of FIG. 2, the shutter 20 has a diaphragm 21 and a shieldingplate 23 that covers the aperture 22 and that is moved by thepiezoelectric actuator 19 transversely to the resonator axis and canthus be moved out of the resonator 13 or into the resonator 13. Themotion can be executed by transverse displacement or pivoting of theshielding plate 23. When a laser pulse train is called up, the controlunit 12 first opens the shutter 16 and then subsequently moves theshielding plate 23 out of the resonator 13, so that the lasingconditions in the resonator 13 are established.

[0020] It is understood that it is also possible for the shutter 20 orthe shielding plate 23 to be disposed between the laser-active medium 10and the end mirror 14 and to be actuated in the same way by thepiezoelectric actuator 19 controlled by the control unit 12. In theexemplary embodiments of the laser beam source in FIGS. 3 and 4, forgenerating laser pulses of constant, unfluctuating pulse energy, anintervention is made not into the resonator 13; instead, a deflectordevice 24 is disposed in the laser beam path from the laser beam sourceto a workpiece 24 that is to be machined, and upon its activation thedeflector device shifts the beam direction of the laser pulses such thatthey do not reach the workpiece 24 but instead reach a so-called beamsump 26, where the pulse energy is dissipated. For calling up laserpulses to the workpiece 24, the activation of the deflector device 25 isdiscontinued again, so that the laser pulses strike the machining site28 on the workpiece 24. Since the laser is already in operation by thetime the first laser pulse is called up to the machining site 28, allthe laser pulses have the pulse energy from the very outset. The layoutof the resonator 13, with the flash lamp 11, the control unit 12, andthe diaphragm 21 located in front of the output mirror 15 and theshutter 16 located behind it is unchanged, so that identical componentsare identified by the same reference numerals.

[0021] In the exemplary embodiment of FIG. 3, the deflector device 25 isembodied as a deflector mirror 27, which can be transferred by anactuator 18 into two pivoted positions. The orientation of the deflectormirror 27 in its two pivoted positions is then done in such a way, inagreement with what has been said above, that the beam direction of thelaser pulses in one pivoted position is aimed at the machining site 28on the workpiece 24 and in the other pivoted position is aimed at thebeam sump 26. Once again, the actuator 18 is preferably embodied as apiezoelectric actuator 19, which needs to adjust the deflector mirror 27by only a small pivot angle. It is understood that it is possible toorient the deflector mirror 27 such that when the piezoelectric actuator19 is unexcited, it assumes the position shown in dashed lines in FIG.3, and is converted into the pivoted position shown in solid lines inFIG. 3 by the piezoelectric actuator 19 that is acted upon by a controlsignal from the control unit 12.

[0022] In the exemplary embodiment of FIG. 4, the deflector device 25 isembodied as an acousto-optical modulator 29. One such acousto-opticalmodulator is described in terms of its layout and mode of operation inthe catalog “Akusto-Optik” [Acousto-Optics], 1999, for instance, put outby ELS Elektronik Lasersystem GmbH, Groβ-Zimmern. Such anacousto-optical modulator comprises a Bragg cell, in which theinteraction between sound and light takes place. The acoustic power isfed into the Bragg cell by means of a transducer. If the Bragg cell istriggered with a suitable high-frequency power, then periodic changes inthe index of refraction develop in the interior of the cell. The laserbeam or laser pulse passing through is diffracted by this change in theindex of refraction and is directed away from the machining site 28toward the beam sump 26. Otherwise, the layout of the laser beam sourceis equivalent to that described for FIG. 3, and so identical componentsare identified by the same reference numerals.

1. A laser beam source for projecting laser pulses, in particular formachining material, having a laser-active medium (10), a pump device(11) for exciting the medium (10), a resonator for inducing oscillationin the medium (10), which resonator has an end mirror (14) and an outputmirror (15), each with an orientation axially parallel to the medium(10), and having a control unit (12) for controlling the number ofpulses, pulse frequency and pulse length, characterized in that one ofthe resonator mirrors (14) is supported pivotably, and the pivotablysupported resonator mirror (14) is engaged by an actuator (18) forpivoting the resonator mirror (14) out of its resonator position and forpivoting the resonator mirror (14) back into its resonator position atselectable times.
 2. The laser beam source as generically defined by thepreamble to claim 1, characterized in that a shutter (20) intervenesbetween the laser-active medium (10) and one of the resonator mirrors(15), and an actuator (18) engages the shutter (20) for closing andopening the shutter (20) at selectable times.
 3. The laser beam sourceof claim 3, characterized in that the shutter (20) has a shielding plate(23), which is actuated by the actuator (18) and shields the resonatormirror (15) from the medium (10) and which is extensible out of theresonator (13) for opening the shutter and is retractable into theresonator (13) for closing the shutter.
 4. The laser beam source of oneof claims 1-3, characterized in that the actuator (18) is controlled bythe control unit (12) in such a way that the pivoting back of theresonator mirror (14) and the opening of the shutter (20) are eacheffected between two excitation pulses of the pump device (11).
 5. Thelaser beam source of one of claims 1-4, characterized in that theactuator (18) has a piezoelectric actuator (19).
 6. The laser beamsource of one of claims 1-5, characterized in that the output mirror(15) of the resonator (13) is followed, in the laser projectiondirection, by a protective shutter (16), and that the protective shutter(16) is triggered by the control unit (12) in such a manner that itopens before pivoting the resonator mirror (14) back again and beforeopening the shutter (20).
 7. The laser beam source as genericallydefined by the preamble to claim 1, characterized in that a controllabledeflector device (25), which upon its activation shifts the beamdirection of the laser pulses, is disposed behind of the output mirror(15) of the resonator (13) in the laser beam path, preferably behind aprotective shutter (16) located behind the output mirror (15).
 8. Thelaser beam source of claim 7, characterized in that the deflector device(25) is embodied as a deflector mirror (27) that can be transferred byan actuator (18) into two pivoted positions.
 9. The laser beam source ofclaim 8, characterized in that the orientation of the deflector mirror(27) in its pivoted positions is accomplished such that the beamdirection of the laser pulses in one pivoted position is aimed at amachining site (28) of a workpiece (24) and in the other pivotedposition is aimed at a beam sump (26).
 10. The laser beam source ofclaims 8 or 9, characterized in that the actuator (18) has apiezoelectric actuator (19) that engages the deflector mirror (27). 11.The laser beam source of claim 7, characterized in that the deflectordevice (25) is embodied as an acousto-optical modulator (29).